A central problem in understanding the development of the brain is to gain insight into the molecular basis for cellular recognition and for how precise neuronal connections are established. During early development axons must pioneer novel pathways through uncharted embryonic landscapes both within the CNS and to and from the periphery. These pathways in turn may serve as a guide for later differentiating neurons, the axons of which have been shown to make highly specific pathway choices and to show selective fasciculation. This process may play a crucial role in the correct wiring of the nervous system. Most hypotheses about the molecular mechanism for selective fasciculation involve specific adhesion or recognition events between axons and/or growth cones mediated by surface macromolecules. However, the challenge has been to test this hypothesis and to identify and characterize such molecules, since the more specific and restricted their expression and distribution is, the lower their abundance. Consequently, a very limited number of molecules has been isolated with proposed adhesion and recognition functions and only a handful of these are confined to subsets of axons and not just involved in general neural cell adhesion. The object of the present proposal is to increase our knowledge of such molecules by determining the molecular structure and function of an antigen recognized by the monoclonal antibodies lan 3-2 and lan 4-2 as well as other antigens which define small subsets of axons forming specific fascicles in the leech. From the amino acid sequence of the antigen, which is a membrane surface glycoprotein, we will analyze the functional implications of its structure. Specifically, we want to test the hypothesis that these antigens are mediating the selective axon fasciculation and thus may represent molecules involved in neuronal recognition and axon guidance. We will also carry out a molecular characterization of the gene locus by exploring its fine structure, its expression and mode of action during development. The promise of cloning these antigens in the leech is that they are specific for a very small and well defined populations of axons and therefore are not likely to be just mediating general adhesion. Our long range goal in analyzing the lan 3-2/4-2 antigen and other leech antigens specific for axons and axonal subsets is to gain basic insights into the functional significance of such molecules, their possible hierarchial organization, functional determinants, and developmental regulation of expression. Since it has been established that many important structural protein sequence motifs have been functionally conserved throughout evolution these investigations should enhance our basic understanding of neuronal recognition and selective fasciculation and provide insights into the underlying causes of aberrant neural connections and abnormal brain development.